Collins Walker Inc. Ductwork Training: Pressure Classes & Water Gauge

Vocabulary practice flashcards covering ductwork pressure classifications, water gauge measurements, positive and negative pressure characteristics, VAV systems, and SMACNA seal classes from Module 3 of the Inside Sales Ductwork Training.

Pressure Class

A duct specification that indicates how hard the fan pushes or pulls on the ductwork, dictating the required sheet metal gauge, reinforcement, joint strength, and sealing.

Water Gauge (w.g.)

A unit of pressure measurement (expressed as in. w.g. or "w.g.) representing how many inches of water column air pressure can push in a manometer, where $1"$ w.g. equals approximately $0.036\,\text{psi}$.

Positive Pressure ($+$)

Duct operating condition where the fan pushes air into the duct, causing the walls to bulge outward; typical of supply air systems where joints and seams must be built not to blow apart or leak.

Negative Pressure ($-$)

Duct operating condition where the fan pulls air out, causing duct walls to collapse inward; typical of return, exhaust, and relief air systems where large rectangular ducts may require extra reinforcement.

$\frac{1}{2}"$ w.g. Pressure Class

A low-range SMACNA pressure class typically specified for small residential installations or light branch ductwork.

$1"$ w.g. Pressure Class

A low-range SMACNA pressure class typically specified for small package units and low-velocity duct systems.

$2"$ w.g. Pressure Class

The standard low-range SMACNA pressure class used for most commercial low-pressure supply/return ducts and ductwork downstream of VAV boxes; serves as the default class on many jobs.

$3"$ w.g. Pressure Class

A medium-range SMACNA pressure class typically used for main supply trunks on VAV systems.

$4"$ w.g. Pressure Class

A medium-range SMACNA pressure class typically used for medium-pressure supply mains located upstream of VAV or fan-powered boxes.

$6"$ w.g. Pressure Class

A medium-to-high-range SMACNA pressure class used for large built-up air handlers, high-rise risers, and special systems.

$10"$ w.g. Pressure Class

A high-range SMACNA pressure class used for industrial, process, laboratory, and special ventilation systems.

VAV Box (Variable Air Volume)

A ceiling unit (also known as a terminal unit or fan-powered box) that throttles airflow to a specific zone, separating medium-pressure upstream ductwork from low-pressure downstream ductwork.

$4"$ Upstream, $2"$ Downstream

A scope letter selection indicating that ductwork is fabricated to medium pressure ($4"$ w.g.) from the air handler to the VAV/terminal boxes, and low pressure ($2"$ w.g.) after the boxes.

Per SMACNA / Per Spec

A scope letter selection used when customer pressure requirements are unknown, directing the shop to fabricate to whatever the engineer's project specification or SMACNA standard mandates.

Seal Class A

The highest SMACNA seal class requiring sealing of transverse joints, longitudinal seams, and wall penetrations; traditionally used for systems at $4"$ w.g. and higher, and frequently required by modern energy codes regardless of class.

Seal Class B

A SMACNA seal class requiring sealing of transverse joints and longitudinal seams; traditionally specified for systems operating at $3"$ w.g.

Seal Class C

A SMACNA seal class requiring sealing of transverse joints only; traditionally specified for systems operating at $2"$ w.g. and below.

Acoustic Duct Liner

An internal duct insulation material commonly required downstream of VAV and fan-powered boxes in low-pressure supply ductwork to attenuate airflow and equipment noise.